1. Field of the Invention
The present invention relates to a piezoelectric thin film usable as an actuator in ink jet recording apparatuses and the like.
2. Background Art
Piezoelectric and ferroelectric thin films represented by lead zirconate titanate (xe2x80x9cPZTxe2x80x9d) thin films can be prepared by sputtering, sol-gel processing, CVD, laser ablation or other processes. In particular, according to the sol-gel process, the composition of the PZT film can be easily controlled. Further, the sol-gel process has an additional advantage that a thin film can be successfully formed by repeating the step of coating a sol composition and the step of pre-sintering the coating. In addition, patterning by photoetching can be carried out during the preparation of a piezoelectric devise by the sol-gel process so that the PZT film is easily formed into a piezoelectric device. An ink jet recording head using a piezoelectric thin film prepared by the sol-gel process has also been proposed (Japanese Patent Laid-Open No. 504740/1993). A film thickness of about 0.5 to 20 xcexcm is in general necessary for a piezoelectric device for an ink jet recording head. Further, in the piezoelectric device for the ink jet recording head, the piezoelectric strain constant is high. The growth of crystal grains by annealing at a temperature of 700xc2x0 C. or above may be necessary to preferably provide such a high piezoelectric strain constant.
However, an attempt to prepare a piezoelectric thin film having a certain film thickness, for example, a film thickness of not less than 1 xcexcm, by the sol-gel process has often cracks in the film during annealing step for the formation of a perovskite structure.
Further, a method in which comprises the steps of coating a sol composition, heating the coating at a high temperature to cause crystallization, and repeating the steps to increase the film thickness has been proposed (e.g., Philips J. Res. 47 (1993) 263-285). So far as the present inventors know, however, the piezoelectric thin film prepared by this method has an interface created by the lamination, i.e., a multilayer structure. The presence of the interface adversely affects piezoelectric properties and often makes it impossible to realize good patterning by etching in the preparation of a piezoelectric device with the thin film.
Therefore, a need still exists for a piezoelectric thin film having good properties and a method for producing the same has been desired in the art.
Accordingly, an object of the present invention is to provide a piezoelectric thin film having good properties, for example, a high dielectric constant and a high piezoelectric strain constant, and a method for producing the same.
Another object of the present invention is to provide a piezoelectric thin film, which does not suffer from cracks at the time of annealing and has no interface created by the lamination, and a method for producing the same.
The present inventors have now found that a piezoelectric thin film having good properties can be provided by modifying the sol-gel process. The present invention has been made based on such finding.
The piezoelectric thin film according to the present invention comprises a thin film with the average grain area of crystal grains observed on the surface of the thin film being not less than 0.1 xcexcm2, the thin film having no multilayer structure in its cross section.
The method for producing a piezoelectric thin film according to a first aspect of the present invention is based on a sol-gel process and comprises the steps of:
a) coating a substrate with a sol composition comprising a sol, of a metal component for constituting a piezoelectric film, and a polymer compound and then drying the coating to form a film;
b) pre-sintering the film to form a porous thin film of gel comprising an amorphous metal oxide;
c) pre-annealing the porous thin film of gel to convert the film to a film of a crystalline metal oxide;
d) repeating the steps a), b), and c) at least once to form laminated films of a crystalline metal oxide; and
e) annealing the films prepared in the step d) to grow crystal grains of perovskite type in the film into a larger size.
The method for producing a piezoelectric thin film according to a second aspect of the present invention is based on a sol-gel process and comprises the steps of:
a) coating a substrate with a sol composition comprising a sol, of a metal component for constituting a piezoelectric film, and a polymer compound and then drying the coating to form a film;
b) pre-sintering the film to form a porous thin film of gel comprising an amorphous metal oxide;
f) repeating the steps a) and b) at least once to form laminated layers of porous thin films comprising an amorphous metal oxide;
cxe2x80x2) pre-annealing the laminated layers to convert the layer to a film of a crystalline metal oxide;
dxe2x80x2) repeating the steps f) and cxe2x80x2) at least once to form laminated films of a crystalline metal oxide; and
exe2x80x2) annealing the films prepared in the step dxe2x80x2) to grow crystal grains of perovskite type in the film into a larger size.